The storage of fluids in closed containers on ships requires pressure relief to allow for changes in pressures in the containers as the fluids contract and expand through diurnal heating and cooling. Compensation for pressure changes in storage tanks by venting is especially important when transporting cargo by sea. As can be appreciated, dangerous high pressure or vacuum differentials between the ship's cargo tanks and the atmosphere could cause container rupture with explosive force, damaging the vessel and even resulting in loss of lives and equipment. At this time fluid cargo shipped by sea is vented by conventional vent valves which have moving mechanical parts which require mechanical maintenance. Also due to the corrosiveness of the sea air and the sea water itself, mechanical failure of these valves due to corrosion is commonplace. As can be appreciated, if the mechanical valves should fail an extremely dangerous situation will exist.
Hydraulic pressure-vacuum valves are known, but heretofore none have been considered for operation on ships at sea. The reason for this is that the motion of ships at sea, especially during storms, is often times greater than 30.degree. from the vertical. In such motion the liquid seal on conventional hydraulic valves is lost resulting in loss of pressure or vacuum in the ship's cargo tanks. Loss of pressure or vacuum in such instances could result in loss or contamination of the fluid cargo being transported.
Therefore it is an object of this invention to provide a hydraulic valve which will maintain a constant pressure or vacuum in a ship's cargo tank, which will also provide for equalization of high pressure differentials between the outside of the container and the inside of the container, which has no mechanical moving parts and which is fail-safe at an angle of greater than 30.degree. from the vertical.